Magnesium duplex metal



Aug. 20, 1935. R. H, BROWN r AL MAGNESIUM DUPLEX METAL Filed Oct. 6,1954 9 4 H "H mm. 0 V. T m fl 1 My WW B Patented Aug. 20, 1935 PATENTOFFICE monssmm DUPLEX METAL Robert H. Brown and Lowell A. Willey, New

Kensington, Pa assignors, by mesne assign- -ments, to MagnesiumDevelopment Corporation, a corporation of Delaware 7 Application October6, 1934, Serial Nb. 147,153

7 Claims. (01. 29-181) This invention relates to duplex metal articlesand is particularly concerned with the production of improved duplexmetals comprising a base of magnesium alloy provided on one or more ofits surfaceswith a coating of a magnesium metal (magnesium or magnesiumalloy).

The commercial application of magnesium base alloy sheet and tubing hasbeen considerably restricted because of their relatively low resistanceto corrosion, particularly in salt atmospheres. Various types of paintand chemical coatings have been used to overcome this dimculty. Coatingsof more corrosion resistant metals, such as zinc and aluminum, have alsobeen suggested for this purpose. These coatings have not, however,provided entirely satisfactory protection, particularly under severecorrosion conditions, such as in the presence of chloride solutions. Theprotection afforded the magnesium base metal by such coatings is apurely mechanical protection which prevents contact of the corrosionmedium with the base metal. Consequently, at any point where thecorrosion medium penetrates the coating, the corrosion of the magnesiumbase metal may proceed unrestricted under the coating. In such case thestructural properties of the article may be substantially impaired, thuscreating an undesirable and at times dangerous condition where theutility of the article is dependent upon the retention of its structuralproperties.

This difficulty is particularly pronounced in duplex metal articleshaving a magnesium alloy base provided with a coating of a morecorrosion resistant metal, such as aluminum or zinc. It has been foundthat at any point where the magnesium alloy base metal is exposed incontact with the coating metal, as, for example, along a cut edge of thematerial, or where the coating has been perforated, an electrolytic typeof corrosion is set up which causes a preferential and acceleratedcorrosion of the magnesium alloy base metal, This preferential orselective attack of the core metal results from the fact that the coremetal has an electrode potential greater than that of the coating metal,whereby galvanic action is set up between the two metals in contact withthe corroding medium, thus causing a flow of current which tends todissolve the core metal. I

It is a primary object of this invention to pro-' vide a duplex metalarticle consisting of a magnesium alloy base and a metallic coating inwhich the magnesium alloy base has an electrode potential lower thanthat of the coating metal, so

that in addition to the mechanical type of protection afforded by thecoating metal, the selective attack of the base metal exposed to thecorroding medium in contact with the coating metal iseliminated. Moreparticularly it is an object 5 of this invention to provide a duplexmetal article in which the coating metal is magnesium or a magnesiumbase alloy and the base metal is a magnesium base alloy having anelectrode potential less than that of the magnesium metal 10 v coating.Another object of this invention is to provide a method for theproduction of a duplex metal article of improved corrosion resistanceconsisting of a magnesium alloy base provided with a coating consistingof magnesium or a magnesium alloy having an electrode potential greaterthan that of the magnesium alloy base.

This invention is based upon the discovery that certain metals have theproperty, when incorporated in magnesium or a magnesium base alloy, ofproducing a. magnesium alloy of lower electrode potential than themagnesium or magnesium alloy to which it has been added. Moreparticularly we have found that by the addition to magnesium or amagnesium alloy of one or more of the class of metals comprising sodium,potassium, barium, calcium, tin, aluminum, zinc, lithium, cadmium,manganese, copper, strontium, bismuth, silver, cobalt, nickel andsilicon, an alloy may be obtained having a lower electrode 30 potentialwithout deleteriously modifying the other properties oi. the alloy,which recommend it as a base metal for a duplex metal article. Thus aduplex metal article may be formed of such magnesium alloy base and amagnesium or magnesium alloy coating which will have substantiallyimproved corrosion-resisting properties, the coating being attacked inpreference to the base metal when exposed to corrosive agents inelectrolytic contact, thus preventing direct corrosive attack on thebase metal and preserving its structural properties.

, While these elements have the common property of lowering theelectrode potential of magnesium and magnesium alloys, they do so invarying degrees, depending upon the particular mettal or metals used, aswell as upon the amount in which they are used. The addition of theseelements may be made in any amount necessary other elements of the basemetal should be such as will not prevent the action of these metals inlowering the electrode potential, as, for example,

by combining with them to form intermetallic compounds which may nothave the property of lowering the electrode potential of the magnesiumalloy.

The choice of the proper alloys to be used in combination in forming theduplex metal articles of our invention depends upon a predeterminedknowledge of the electrode potential of magnesium and its variousalloys. These diiferent potentials are readily measured in accordancewith the methods long known in the art against a standard electrode; Ifthe electrode potential of the magnesium metal coating iselectronegative with respect to the standard electrode, it is notnecessary that the magnesium alloy base be electropositive with respectto the standard electrode but only that the coating metal be moreelectronegative than the base metal. Therefore, throughout thisspecification and the appended claims, when it is said that theelectrode potential of. the base metal should be lower than that of thecoating metal, it is meant that the base metal should be lesselectronegative with respect to the standard electrode than is thecoating metal.

While no fixed potential difference is required between the magnesiumalloy base and the coating metal, it is desirable that there be arelatively wide difference in the electrode potential of the two layers,since with greater differences in electrode potential there usuallyoccurs less attack on the magnesium alloy base, and by suitable vinwhich the coating metal has an electrode potential of the order of 0.01to 0.1. volts greater than the electrode potential of the magnesiumalloy base are particularly resistant to galvanic corrosive action.

A convenient standard electrode for measuring the potential diiferencemay be readily selected from those now in regular use for similarmeasurements. For the purpose of this description of thednvention, acalomel electrode has been selected as a standard. In measuring theelectrode potential in question, the magnesium metal to be tested ismade one element of an electrolytic cell; the calomel electrode is madethe other 1 element. As electrolyte, a normal solution of sodiumchloride containing 0.3 per cent by "weight of hydrogen peroxide may beused. 'The circuit is closed and the potential difference of theelements "is measured on a potentiometer. The calomel.

electrode used is made up in the usual way of mercury in contact withmercurous chloride, and in contact with this a it; normal potassiumchloride solution saturated with mercurous chloride.

In .forming the duplex metal article, any of the known methods offorming such duplex metal articles may be used. One method which we havefound particularly useful is to roll together the coating alloy and thebase alloy in sheet or slab form. It is satisfactory to castthe-base'metal in an ingot mold lined with the coating alloy. The

article may then, if desired, be mechanically worked, as, for example,by rolling or drawing or other suitable operation to form the particulartype of duplex metal article-desired, such as sheet, I

rod or other form of material. The working with intermediate heatingincident to such forming operation generally tends to improve the bondbetween the coating metal and the base metal. Duplex material may alsobe produced by spraying or otherwise placing the coating metal on thebase metal. This method may be used to advantage where irregular shapes,such as castings, are to be coated.

. The duplex metal articles of our invention may be provided on any oneor all exposed surfaces with the protective coating alloy, as, forexample,

. i shown inFigs. 1 and 2 of the drawing. Fig. 1

shows a duplex metal article comprising a base of magnesium metal Iprovided on its top and bottom surfaces with a magnesium alloy coating 2a d 2 a base 3 of a magnesium metal provided on. its inner surface witha magnesium coating metal t. This type of material is useful, forexample, in'the manufacture of pipe used to conduct a material corrosiveto the structural alloy.

We claim:

' l. Aduplex metal article consisting of a base of a magnesium alloycontaining at least one of the metals of the class consisting of sodium,potassium,-nickel, cobalt, barium, calcium, bismuth, strontium, silver,aluminum, manganese, tin, zinc copper, lithium, cadmium and silicon, andprovided with' a coating of a magnesium metal, said magnesium alloy basehaving an electrode potential lower than that of the magnes'ium metalcoating, 'as determined by reference to a standard electrode. 1 w

2. A duplex metal article consisting of a base of a magnesium alloycontaining at least one of the metals of the class consisting of sodium,potassium, nickel, cobalt, barium, calcium, bismuth, strontium, silver,aluminum, manganese, tin, zinc, copper, lithium, cadmium and silicon,and provided with a-coating of a magnesium metal, said base havingane'lectrode potential lower than that of the magnesium metal coatingand lower than that of magnesium, as determined by reference to standardelectrode.

3, A duplex metal article consisting of a'base of a magnesium alloycontaining at least one of the metals of the class consisting of sodium,potassium, nickel, cobalt, barium, calcium, bismuth, strontium, silver,aluminum, manganese, tin, zinc, copper, lithium, cadmium and silicon,and provided with a coating of magnesium, said base having an electrodepotential lower than 'magnesium, as determined by reference to astandard electrode.

4. A duplex metal article consisting of a base of a magnesium alloycontaining at least one of the metals of the class consisting of sodium,

potassium, nickel, cobalt, barium, calcium, bis muth, strontium, silver,aluminum, manganese,

tin, zinc, copper, lithium, cadmium and silicon,

and provided with a coating of a magnesium al- 10y, said base having anelectrode potential lower than that of the magnesium alloy coating, andsaid magnesium alloy coating having an electrode potential less thanthat of magnesium, as determined by reference to a standard electrode. V

' 5. A duplex metal article consisting of a base of a magnesium alloycontainingat least one of the metals of the class consisting of sodium,potassium, nickel, cobalt, barium, calcium, bismuth, strontium, silver,aluminum, manganese,

In Fig. 2 is shown, in cross section, a duplex metal article in tubularform comprising tin, zinc, copper, lithium, cadmium and silicon, andprovided with a coating or a magnesium alloy, said base having anelectrodepotential lower than that oi the magnesium alloy coating, andsaid coating having an electrode potential lower than that of magnesium,as determined byreierence to a standard electrode.

6. A duplex metal article consisting of a base oi a magnesium alloycontaining at least one of the metals of the class consisting of sodium,potassium, nickel, cobalt, barium, calcium, bismuth, strontium, silver,aluminum, manganese, tin, zinc, copper, lithium, cadmium and silicon,and provided with a coating 01' a magnesium metal, said magnesium alloybase having an electrode potential 0.01 to 0.1 volts lower than that ofthe mgnesium metal coating, as determined by reference to a standardelectrode.

electrode potential about 991 to 0.1 volts lower 10 than that o! themagnesium coating metal as measured by reference to a standardelectrode.

ROBERT H, BROWN,

LOWELL A. WILLEY. 15

